Highly Anisotropic in-Plane Excitons in Atomically Thin and Bulklike 1T′-ReSe₂

Abstract

Atomically thin materials such as graphene or MoS are of high in-plane symmetry. Crystals with reduced symmetry hold the promise for novel optoelectronic devices based on their anisotropy in current flow or light polarization. Here, we present polarization-resolved optical transmission and photoluminescence spectroscopy of excitons in 1T'-ReSe. On reducing the crystal thickness from bulk to a monolayer, we observe a strong blue shift of the optical band gap from 1.37 to 1.50 eV. The excitons are strongly polarized with dipole vectors along different crystal directions, which persist from bulk down to monolayer thickness. The experimental results are well reproduced by ab initio calculations based on the GW-BSE approach within LDA+GdW approximation. The excitons have high binding energies of 860 meV for the monolayer and 120 meV for bulk. They are strongly confined within a single layer even for the bulk crystal. In addition, we find in our calculations a direct band gap in 1T'-ReSe regardless of crystal thickness, indicating weak interlayer coupling effects on the band gap characteristics. Our results pave the way for polarization-sensitive applications, such as optical logic circuits operating in the infrared spectral region.

Pressure dependence of direct optical transitions in ReS2 and ReSe2

“…These values are lower than the experimentally measured optical gap, of 1.31 eV and 1.36 eV. Note that, since the exciton binding energy is ≈ 120 meV, 24 the experimental fundamental gaps are = 1.43 eV and 1.47 eV, as obtained from = + . Hence, the calculated fundamental band gap is ≈20% lower than the experimental.…”

In‐Plane Isotropic/Anisotropic 2D van der Waals Heterostructures for Future Devices

“…This quasi‐1D performance leads to the formation of high binding energy quasi‐1D excitons and trions in 2D systems. The binding energy of such quasiparticles are usually found higher than those of quasiparticles in many in‐plane isotropic 2D semiconducting materials . In 2D isotropic materials, especially in various monolayer TMDs (MoS 2 , MoSe 2 , WS 2 , and WSe 2 ), binding energies of 18–36 meV for trions and 380–710 meV for excitons have been reported .…”

“…For TMD materials, MoS 2 , MoSe 2 , WS 2 , WSe 2 , and MoTe 2 exhibit in‐plane isotropic optical and electrical behaviors . However, BP and its isoelectronic group IV MNs (SnS, SnSe, GeS, GeSe), ReSe 2 , ReS 2, and 2D perovskites show anisotropic behavior due to their puckered crystal lattices . Hence, strong light–matter interactions and reduced dimensionality lead to the formations of quasi‐2D excitons and trions in isotropic materials, and quasi‐1D excitons and trions in anisotropic materials .…”

Ultrafast charge transfer and vibronic coupling in a laser-excited hybrid inorganic/organic interface

“…In the Si-based system considered here, excitons are not expected to play a significant role. However, this is not the case of hybrid interfaces composed of TMDCs, where screening and excitonic effects are known to be very relevant [26,[141][142][143][144][145][146] also in the dynamical regime [147][148][149][150][151][152]. Combined efforts in the aforementioned directions will offer unprecedented insight into the fundamental mechanisms of charge-transfer dynamics in hybrid interfaces, paving the way for controlled manipulation of lightmatter interaction in this class of materials on the atomistic space and time scales.…”